This invention relates to a vibration and shock isolation system for use with medical imaging equipment, and in particular, to a system suitable for protecting such equipment from damage in mobile military applications.
In medical computed tomography (CT), a cross-sectional image of a patient is generated by computer processed data collected by a CT machine. The CT machine employs an x-ray source collimated so as to form a fan beam of x-rays. The fan beam is directed through the patient along a fan beam plane generally parallel to the cross section of the image to be produced. A series of detectors, positioned within the fan beam plane, receive the radiation after it passes through the patient and provide a series of intensity measurements along a number of rays from the x-ray source to each detector element. After one such projection is obtained, the x-ray source and detector are rotated about the patient to a new position and a new projection is taken. Multiple projections taken at different angles form a projection set which may be "reconstructed" into the tomographic image. The patient is normally supported on a radiolucent table which extends through the fan beam plane and which may be moved to a variety of positions within the fan beam plane.
The x-ray source and x-ray detectors are mounted on a gantry for rotation about the patient to obtain the projection set as described above. The rotating gantry may also hold an x-ray tube cooling system and certain detector electronics and thus may have considerable mass. The gantry is mounted to a rigid support frame constructed to hold the gantry precisely within a single plane and thus to ensure the integrity of the projection set data.
The entire CT machine, including the table but excluding the remote console used for controlling the CT machine and for receiving and displaying the tomographic images, may weigh on the order of 2.5 tons. Nevertheless, the CT machine is a sensitive instrument which must be protected from shocks and vibration. The mathematics of image reconstruction requires that the x-ray tube and detectors be precisely aligned during the rotation of the gantry to avoid imaging artifacts, i.e., errors in the reconstructed image visible as obscuring rings or streaks. Accordingly, care must be taken that the gantry runs true without deviation or wobble.
Large shocks may distort the gantry or its supports and may damage electronic subcomponents such as the x-ray tube. Lower amplitude but continuous vibrations may cause misalignment of the x-ray tube and the detectors, so as to reduce the image quality, or may cause premature failure of the CT system's numerous electronic components. Some form of mechanical isolation is critical for a CT system that is not in a fixed site in a stable environment.
While the physics of shock protection and vibration isolation are generally understood, the isolation of a CT machine represents a considerable challenge because of both its large mass and its sensitive construction. Adapting a CT machine for use in a military environment or the like, to be transported in areas having only unimproved or damaged roads, requires a high degree vibration isolation and protection against impulse shocks. For example, in a military mobile hospital, the CT machine may be routinely subject to 12" drops.
It may be expected that the primary shocks to the CT system during transportation will be directed along a vertical axis; however, this cannot be guaranteed. Conventional techniques for cushioning a load against vertical and rotational shocks may require complex arrangements of multiple shock absorbing elements positioned along different axis and both adding to the complexity of the isolation system and reducing its reliability.
Ultimately, the complex nature of the interaction between the CT structure and the shock isolation system under a variety of shocks requires that the isolation system be fine tuned to particular CT system and shelter being used. Such tuning is difficult with typical metal spring isolators.